There is increasing interest in constructing hollow structures such as culverts and tunnels from prefabricated sections which can often increase the speed of construction and lower costs. Sections made from corrugated steel have been used for this purpose, but more recently there has been great interest in the use of sectional concrete structures, as for example in U.S. Pat. No. 3482 406 and European Pat. No. 81402.
In the design of arched or vaulted constructions incorporating prefabricated concrete sections, the shape and thickness of the sections must be such that under the design load, which will normally be a static load, the section is under compression at all times. In general, the optimal shape of the curve will be the so-called antifunicular of loads or an approximation thereto. The overall dimensions of the section depend largely on the requirements of the total structure and it can be seen that there is a virtually infinite range of sizes, thicknesses and cross-sections which are required to optimize the design of the wide range of structures which may be needed to suit different circumstances.
This situation has made the prefabrication of the sections relatively expensive. Where large structures are concerned, several circumferentially arranged sections will be required to complete the structure in order to permit these to be transported without undue difficulty. However, it is scarcely possible to design a structure of a particular size and shape to be used for all purposes since, for example, the load, e.g. depth of overfill above any particular tunnel or culvert, will depend entirely on the circumstances of the job and this will determine the optimal dimensions of the structure.
One solution to this problem is to use excessively thick prefabricated sections which will sustain a wide range of loads; it will readily be appreciated that this inevitably increases the cost not only of materials but also of handling and installation. Another possibility would be to design the sections to the optimal thickness and shape and to use a very large number of moulds each specially created for the structure concerned. Again this would be an expensive procedure. We have now found, however, that such prefabricated concrete sections can be made economically using moulds of variable curvature such that each mould is capable of forming a wide range of concrete sections of calculated optimal curvature and size.
British Patent specification No. 668372 describes an apparatus for the construction on site of an arched concrete roof wherein shuttering plates onto which the concrete of the arch is applied are supported by a beam of fixed curved appropriate to the design of the arch, the beam being supported by pillars of variable height. The shuttering plates overlap at their edges but are otherwise not attached to each other, so that after the concrete arch has set, the shuttering plates may be removed individually from below the arch. There is no suggestion of using such a system to construct prefabricated sections of a concrete arch and because of the use of a supporting beam of fixed curvature, the system could not be used to construct prefabricated sections of variable curvature suitable for prefabrication of a wide range of arch designs.